1. Field of the Invention
The present invention relates to a heat generating resistive element of a liquid discharge head (which may be hereafter called an ink jet head or a recording head) for recording or printing letters, marks and images on a recording medium containing paper, a plastic sheet, fabric and an article, by discharging a functional liquid such as ink; a substrate for the liquid discharge head having the heat generating resistive element; the liquid discharge head; and a manufacturing method therefor.
2. Related Background Art
An ink jet recording device with the use of this kind of ink jet heads (recording heads) has the characteristic of recording an image of high definition at high speed, by discharging ink of fine droplets from a discharge port at high speed. Particularly, an ink jet recording device which employs an electrothermal conversion body for an energy-generating device for generating energy used for discharging ink, and has the system of discharging an ink bubble produced by the heat energy generated by the electrothermal conversion body, is suitable for the higher-definition and higher-speed recording of images, and the miniaturization and colorization of a recording head and a recording device, so that it has attracted a great deal of attention in recent years. (For instance, see U.S. patent application Ser. Nos. 4,723,129 and 4,740,796)
FIG. 1 is a schematic plan view showing the general configuration of an essential part of a substrate in a recording head used for ink jet recording as described above. FIG. 2 is a schematic sectional view of a substrate 2000 for an ink jet recording head cut along a 2—2 line corresponding to an ink channel in FIG. 1.
As shown in FIG. 1, a plurality of discharge ports 1001 are arranged in this ink jet recording head, and an electrothermal conversion element 1002 generating a thermal energy used for discharging ink from each discharge port 1001 is installed on a substrate 1004 of each ink channel 1003. The electrothermal conversion element 1002 mainly comprises a heat generating resistive element 1005, an electrode interconnection 1006 for supplying an electric power thereto, and an insulating film 1007 for protecting them.
Each ink channel 1003 is formed by jointing a top board having a plurality of channel walls 1008 integrated therein to a substrate 1004 while aligning a relative position of it to an electrothermal conversion element 1002 on a substrate 1004 by a method such as image processing. Each ink channel 1003 has the end on the opposite side of a discharge port 1001 communicating with a common liquid chamber 1009, in which the ink supplied from an ink tank (not shown) is stored.
The ink supplied to the common liquid chamber 1009 is led to each ink channel 1003, forms a meniscus in the proximity of a discharge port 1001, and is retained there. At this moment in time, an ink head selectively drives an electrothermal conversion element 1002, rapidly heats the ink on a thermic effect surface to boil it with the use of the generated heat energy, and discharges the ink through an impulse force generated by boiling.
Referring to FIG. 2, a substrate 2000 for an ink jet recording head is constituted by multilayers consisting of a silicon substrate 2001, a heat storage layer 2002 made of a thermal oxide film, an interlayer film 2003 which also serves as a heat storage function and is made of a SiO film or a SiN film, a heat generating resistive layer 2004, metal wiring 2005 made from Al, Al—Si and/or Al—Cu, a protective layer 2006 made of a SiO film and a SiN film, and a cavitation resistant film 2007 for protecting the protective layer 2006 from chemical and physical shocks caused by the heating of the heat generating resistive layer 2004. The substrate 2000 for the ink jet recording head has a thermic effect portion 2008 of the heat generating resistive layer 2004 configured on one part of the top face.
The heat generating resistive element used in such a recording head is required to have characteristics in the following:
(1) being superior in thermal responsiveness and capable of instantly discharging ink;
(2) having slight change in an ohmic value after a continuous driving operation at high speed, and capability of stabilizing the bubbling state of ink;
(3) having superior heat resistance and thermal stress properties, the long life, and high reliability.
As a heat generating resistive element satisfying these requests, the configuration of using TaN for the material of the heat generating resistive element is disclosed in Japanese Patent Application Laid-Open No. H07-125218.
The stability of the characteristics of a TaN film constituting a heat generating resistive element, particularly the rate of change in resistance after the heat generating resistive element has repeated a recording operation over a long period of time, has a strong correlation with a composition of a TaN film. When tantalum nitride including TaN0.8 hex among the TaN films constitutes a heat generating resistive element, the heat generating resistive element shows a low rate of change in electrical resistance after having repeated the recording operation over a long period of time, and superior discharge stability.
However, as described above, an ink-jet recording device has been increasingly required in recent years to have higher functions such as higher picture quality and recording at higher speed.
Among them, as for the improvement of picture quality, the picture quality can be improved by decreasing the size of a heater (a heat generating resistive element) and by reducing a dot size (reducing a discharge quantity per dot). In addition, as for the higher-speed recording, the speed of recording can be increased by driving a heater in shorter pulses than before and consequently increasing a driving frequency.
However, in order to drive a heater at high frequency in an ink jet head configuring a reduced size of a heater for coping with higher picture quality as described above, the heater (a heat generating resistive element) is needed to increase a sheet resistance value.
FIGS. 3A and 3B are views for describing a relation between a heater size and various drive conditions. FIG. 3A shows the relation between the sheet resistance value of a heat generating resistive element and the value of an electrical current with respect to a driving pulse width, when a heater size changes from a large size (A) to a small size (B) while a driving voltage is constant. In addition, FIG. 3B shows the relation between the sheet resistance value of a heat generating resistive element and the value of an electrical current with respect to the driving voltage, when the heater size changes from the large size (A) to the small size (B) while the driving pulse width is constant. In views 3A and 3B, a solid line shows the sheet resistance value, and a broken line shows the value of the electrical current.
As is evident from FIGS. 3A and 3B, in order to drive an ink jet head having the decreased size of a heater in the same condition as a conventional one, the sheet resistance value has to be increased. In addition, a method for driving by increasing the sheet resistance value and the driving voltage reduces the value of an electrical current in relation to energy, so that it can save energy. Particularly, in the case of a liquid discharge head arranging a plurality of heat generating resistive elements therein, the effect becomes great.
However, HfB2, TaN, TaAl or TaSiN, which have been conventionally used for the material of a heat generating resistive element in the ink jet recording head as described above, show a specific resistance value of about 200 to 800 [μΩcm]. In consideration of stably manufacturing the heat generating resistive element and stabilizing the discharge characteristics of a liquid, the maximum film thickness of the formed heat generating resistive element is limited to about 400 angstrom, but the sheet resistance value obtained when employing the above-described materials is limited to about 200 [Ω/□]. Consequently, it is difficult to obtain a higher sheet resistance value than the above value so long as using the above-described material.
From this reason, conventionally, there has not been such a heat generating resistive element for an ink-jet recording head as to have superior thermal responsiveness in short pulse drive and a high sheet resistance value. Furthermore, when making a heater size small and the ink-jet recording head discharge small ink drops in order to obtain a recording image of higher definition, a conventional heat generating resistive element has to increase the value of an electrical current passing through it, and consequently have caused the problem of increasing heat generation and consequent energy consumption.